I have to run a for loop in blender, but screen is not being updated
This is how scripts are intended to be run in Blender. You should not attempt to update the interface while running a script - that can cause problems with other processes in Blender. 

If you need some functionality that runs in Blender while the rest of Blender is functioning at the same time you should make a modal operator. There are a few examples included with Blender that you can reach from the Text Editor's header menu Templates - > Python -> Operator Modal... :